The instruments that meaasures rotational speeds of motors and generators are called the tachometers.

Following are the different kinds of tachometers:

I.Mechanical tachometers

II.Electrical tachometers

1. C. tachometer generators
2. C. tachometer generators
3. Photo-electric tachometers (or Speed-meter)
4. Toothed motor variable reluctance tachometer etc. (Electrical tachometers are favored in comparison to mechanical tachometers because the electrical transducers produce greater advantages)

Following are the tachometer generators that are termed below:

1. D.C. Tachometer Generator:

In a D.C. generator the e.m. fen gendered depends on the following factors:

1. Field excitation
2. Speed

The permanent magnet pole pieces that are used in the field system. It produces a certain amount of voltage that depends on the speed. Hence the emf generated can be easily tracked by the speed.

A D.C. tachometer generator is illustrated in Fig. 32 given below:

The illustrated model consists of:

1. A shaft whose speed will be measured after coupling it to the armature.
2. A coil which moves and called voltmeter which is linked with the brushes to estimate the generated voltage. There is a variable resistance R which is combined to control the current via voltmeter.

So the voltage is proportional to speed, and it can be standardized only with speed (r.p.m.)

1. A.C. Tachometer Generator:

Due to certain disadvantages of the D.C. tachometer generator, the A.C. tachometer generator was being created. The commutator and brushes are removed in A.C. tachometer generator, and this makes it different from the D.C. generator.

An A.C. tachometer generator is diagrammatically represented in Fig. 33 given below.

The above-given figure consists of the following features:

• It includes an alternator, a stationary armature (stator) and a rotating field system (rotor). In this A.C. generator, there are no commutation problems do exist.
• The changing e.m.f. is made to convert into a stationary coil and is then corrected, and then the output is obtained in the form of D.C. voltage, measured with a moving coil voltmeter (V).
• The ripples of the received voltage are further rectified by the capacitor filter (C).

Therefore to measure speed either the amplitude of the voltage or the frequency or both can be utilized. In an A.C. tachometer, the induced voltage is thus the desired parameter.

1. Photo-electric Tachometer:

Fig. 34 thus represents the photoelectric tachometer.

Fig. 34

• Now this includes an opaque round plat form that is fixed to a movable shaft whose speed is our preference for measuring purpose. This disc has many holes around the periphery which are all the same. At one end of the disc, there is a light source (L) while there is a light sensor present at the other end of the disc. The highlighting part is that both of these are placed in a straight line.
• When the disc rotates, the holes and opaque portions in the disc alternatively faces the light source and the light sensor. So when the hole is in between,then the light travels up to the sensor through the holes, as a result of which an output pulse is thus generated.

But when the opaque part comes in between the light source and the sensor then the light gets hindered resulting in no pulse output. Therefore it can be concluded that whenever a hole comes in line with the light source and sensor, only then a pulse is thus generated. These pulses are then measured through an electronic device.

The number of pulses depends on:

1. The number of holes in the disc
2. The speed of the rotating shaft

However, as a matter of fact,the number of holes in the disc are fixed, so the amount of pulse created depends on upon the speed of the shaft. The electronic device that counts these pulses is standardized by (r.p.m.)

Advantage. It is a digital instrument.

Disadvantage. The light source has to change periodically, and if the grating period is made least,then there is a chance of getting erroneous output.

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